After Real Madrid signed defenders Eder Militao and Ferland Mendy, 21-year-old left-back Theo opted to make a €20million switch to Milan, bringing an end to his two-year spell with Los Blancos.
Theo cost Zinedine Zidane's team €30m when he arrived from cross-city rivals Atletico Madrid in July 2017, but he made just 13 LaLiga appearances for the Santiago Bernabeu club.
He also played three times for Real Madrid in the Champions League, helping the club to win the competition in 2017-2018, and he told Milan's official website he hopes for more success in Europe with the Rossoneri.
"I'm really excited and happy to be joining AC Milan," said Theo. "I hope I can make history with this club.
"I'll give my very best for this club. AC Milan is a great club – that is why I have come here, to reach the very top and win the Champions League with the Rossoneri.
"I would like to tell you [the fans] that I'm here to make history and do my very best for you all.
"I'm delighted to be here. Forza Milan!"